The present invention relates to spark plugs for high power high energy ignition systems for use in internal combustion engines with difficult-to-ignite dilute mixtures, such as lean mixtures and high exhaust residual or high EGR mixtures. High power ignition systems delivering 100's of watts of power for a time duration of 0.2 to 2 millisecond (msec) increase the engine's tolerance for dilute operation for more efficient and cleaner combustion.
To produce the high spark power of typically 50 to 500 watts high current arc type spark discharges are required. Arc discharges are also required to avoid spark break-up or spark segmentation at high air-flows which are favored as they increase the engine's tolerance for dilution and increase the bum rate. More specifically, an arc discharge in the 1 to 10 amps range maintained across a wide spark gap of 1.5 to 3 millimeters or greater provides the 50 to 500 watts of required power and the tolerance to high bulk flows of 20 meters/second (m/sec) at the spark plug site without spark segmentation. A hybrid single or dual discharge type ignition, disclosed in PCT patent application Ser. No. 94/12866 (including U.S. designation), provides such an arc discharge with the required spark power of 50 to 500 watts and the required spark duration of 0.2 to 2 milliseconds without spark segmentation or break-up under high flow conditions.
Such an arc discharge places high stress on the spark plug in terms of erosion, fouling, and over heating of the spark plug firing end. Conventional spark plugs with standard material "J" ground electrodes, or even multiple ground electrodes, erode far too quickly under arc discharge operation to be useful, and surface gap plugs short out too quickly. More advanced circular gap spark plugs can last longer but cannot meet the new goals of even longer spark plug life without compromising other important ignition characteristics.
Conventional glow discharge ignitions, which produce relatively little erosion at the spark plug (versus the arc discharge), provide only 5 to 25 watts to the mixture, and high energy ignition, HEI, supplies only twice that amount, less than the required 100's of watts of power. Moreover, under conditions of moderately high flow as found in some modern engines, the spark discharge of even the HEI system is broken-up, or segmented, to compromise igniting ability. Variants of HEI systems which use alternating current (AC) sparks and provide low plug erosion, perform even worse under conditions of bulk flow since they already provide, by definition, an undesirable segmented spark.
It is therefore desirable to employ an ignition system that can supply the required 100's of watts of ignition power in the form of a single polarity arc type spark discharge resistant to spark segmentation under high bulk flow conditions of 5 m/sec and greater, and to employ a spark plug that can withstand the higher required spark currents as well as the higher flow conditions with acceptable electrode erosion, without spark plug fouling, without electrode interference or quenching of the initial flame front, and without absorbing excessive combustion heat from the high temperatures that exist at the spark plug site.
Circular or toroidal gap spark plugs are best suited for this application. Early versions are disclosed as part of higher power ignition systems in U.S. Pat. Nos. 4,677,960, 4,774,914, 4,841,925, 5,207,208, 5,131,376, 5,211,147, and 5,315,982 which are of common assignment with this patent application with Dr. M. A. V. Ward as a sole or joint inventor (and are incorporated herein by reference as though set out at length herein). However, these and other circular gap spark plugs, disclosed in other patents, have large high heat capacity flame quenching electrodes, are subject to spark plug fouling by electrode material being deposited on the spark plug insulator nose, have a relatively recessed spark, or require firing to the piston at some or all of their operating conditions to improve their operation.